Back-scatter absorption gas imaging (BAGI) is a powerful technique for imaging gaseous species that are transparent in the visible wavelength band. Back-scatter absorption gas imagers generally include a source of spectrally pure light that is tuned into an absorption feature of the gas to be detected, and a detector for detecting light scattered from a target area. The detector may be a camera or some other imaging device for displaying an image of the target area. In use, the target area is illuminated with the spectrally pure light and the detector is positioned to detect any light which is back-scattered. If no gas is present, the light is back-scattered from the target and collected by the detector. In this case, a clear image of the target scene is displayed. In contrast, if gas is present then it will absorb the light and a dark area corresponding to the gas will appear on the image. In this way there is provided a simple and effective visual means for detecting gas.
The application of BAGI to hydrocarbon and in particular methane detection depends on the availability of an optical source that produces light in the mid-infrared band particularly over the wavelength range of 2-6 microns, where these gases exhibit strong absorption lines. The absorption linewidth associated with discrete absorption features in a gas such as methane is typically of the order of ˜1 GHz due to Doppler broadening at low pressures. As the pressure is increased to normal air pressure, the linewidth increases to ˜5 GHz due to pressure broadening. To detect such a single line absorption feature effectively it is therefore necessary to use an optical source that exhibits a spectral linewidth comparable to or less than this latter figure. Currently, there are no lasers that operate in the required wavelength range and have a suitable linewidth and tunability capability, whilst emitting sufficient power for practical use in back-scatter absorption gas imaging. Optical parametric oscillators (OPOs) are therefore employed in order to convert the wavelength from a parent pump laser operating at a wavelength that is shorter than required, into the spectral band of interest, that is to say frequency down-conversion. Optical parametric oscillators include a nonlinear medium that is operable to generate a signal and an idler wave in response to being stimulated with a laser pump wave source. In conventional OPOs, the pump wave source resides in a separate optical cavity to that of the nonlinear medium. These will be referred to as extra-cavity OPOs.
The current state of the art in the application of OPO's to BAGI has been developed by an American research group based at Sandia National Laboratories (SNL). This group has published numerous papers in this field, such as “Backscatter Absorption Gas Imaging—a New Technique for Gas Visualization” by T. G. McRae, and T. J. Kulp, Applied Optics, 1993. 32(21): p. 4037-4050; “Development of a pulsed backscatter-absorption gas-imaging system and its application to the visualization of natural gas leaks” by T. J. Kulp et al., Applied Optics, 1998. 37(18): p. 3912-3922; “Active infrared imagers visualize gas leaks” by T. J. Kulp and T. McRae, Laser Focus World, 1996. 32(6): p. 211; “Portable laser-based imager offers efficient hydrocarbon detection” by T. J. Kulp et al, Laser Focus World, 1004. 40(3): p. 93-; and “Demonstration of differential backscatter absorption gas imaging” by Powers, P. E. et al, Applied Optics, 2000. 39(9): p. 1440-1448. In general, the SNL group has employed both continuous-wave and pulsed OPO's for the illumination source and focal-plane array cameras and rastering scanners for the imaging system. Although impressive results have been reported in terms of both the lower detectability limit and target range, the technology developed at Sandia National Laboratories is characterised by very high component costs, high system complexity, prohibitive power requirements and bulky designs. Such issues cast serious doubt over the possibility of developing such systems into truly portable ambulatory devices appropriate for field use.
Another OPO based BAGI technique is described in “Hyperspectral imaging of gases with a continuous-wave pump-enhanced optical parametric oscillator” by D. J. M. Stothard, M. H. Dunn, and C. F. Rae, Optics Express, 2004. 12(5): p. 947-955. This arrangement uses a pump enhanced continuous wave OPO. Whilst this arrangement provides an effective means for detecting gas, the optical requirements are such that the device is difficult to make fully portable. Clearly, this is a significant disadvantage.
An object of the present invention is to provide an improved back-scatter absorption detector/imager.